1. Field of the Invention
The present invention relates to a technique of detecting a pulldown sequence from input interlaced images.
2. Description of the Related Art
A progressive scanning display such as a liquid crystal display (LCD) or a plasma display panel (PDP) is increasingly used in a television or a personal computer instead of a commonly used cathode ray tube.
In a progressive method, which is widely used in the progressive scanning display mentioned earlier, all the lines (scan lines) that form a frame are sequentially drawn, thus enabling to display an image with less flickering. For example, the progressive method of 24 frames per second (fps) is used in a cinema image such as a movie.
In a method called interlaced method, a field that is a convergence of odd lines and a field that is a convergence of even lines are alternately drawn. For example, a national television standard committee (NTSC) method which is an interlaced method of 30 fps (60 fields) is used in a television broadcast.
In a television which includes the progressive scanning display mentioned earlier, an interlace to progressive (IP) conversion process is carried out to convert an image of the interlaced method into an image of the progressive method. Thus, the image of the interlaced method (an interlaced image) such as the image in the television broadcast is displayed with a high image quality.
However, the interlaced image includes two types of images, an original image of the interlaced method and an image that is converted from the cinema image etc. For example, for carrying out the television broadcast from the 24 fps cinema image, the television, which includes the progressive scanning display, receives an image (a telecine image) converted to the 30 fps (60 fields) interlaced image. The received telecine image is IP-converted into the progressive image and displayed.
In a process called 2:3 pulldown process, the progressive image such as the 24 fps cinema image is converted into the 30 fps (60 fields) interlaced image. In a process called 2:3 pulldown sequence detection, the interlaced image subjected to the 2:3 pulldown process is reversely converted (IP-converted) into the original progressive image.
In the 2:3 pulldown process, for converting the 24 frame progressive image into the 30 frame (60 fields) interlaced image, a field before two fields is repeated once in a five field cycle (repeated field) to realize the conversion.
In the 2:3 pulldown sequence detection, by detecting a regularity of the repeated field, the interlaced image subjected to the 2:3 pulldown process is IP-converted into the original progressive image.
Thus, a detection of the repeated field in the 2:3 pulldown sequence detection is extremely significant for generating the high quality image using the IP conversion process. Due to this, various technologies have been disclosed for accurately detecting the repeated field.
For example, in a technology disclosed in Japanese Patent Application Laid-open No. 2003-179884, a frame difference and a field difference in a small area are compared to predetermined threshold values to detect the repeated field and to carry out the IP conversion process (2:3 pulldown sequence detection).
Further, in a technology disclosed in Japanese Patent Application Laid-open No. H3-58677, a movement is detected from a difference absolute value by using the frame difference of a current field and a field before two fields, the difference absolute value is compared to the predetermined threshold values, and the repeated field is detected to carry out the IP conversion process (the 2:3 pulldown sequence detection).
A comparison of the frame difference (a feature quantity) and the threshold values is explained in detail. As shown in FIG. 9, generally, when using the 2:3 pulldown process to convert the 24 frame cinema image into the telecine image of 60 fields, two fields or three fields are alternately created from a single frame.
In other words, fields 1 and 2 are created from a frame A. Fields 3, 4, and 5 are created from a frame B. Similarly, two fields are created from a frame C and three fields are created from a frame D. A field before two fields is substituted as the repeated field once in the five field cycle. In other words, fields, which are the same as the field 5 and the field 8, are substituted respectively in the five field cycles in a total of ten fields to realize the conversion.
Next, the repeated field is detected to reversely convert the 60 fps interlaced image (the telecine image) into the 24 fps progressive image. To be specific, each field is compared with the field two fields before the field, and the difference absolute value of the feature quantity (the field difference) that is a comparison result for each of the pixels constituting the field is plotted in a chart shown in FIG. 10. As shown in FIG. 10, the difference absolute value of the feature quantity (the field difference) is smaller for the fields created from the same frame and a correlation between the fields is larger. The difference absolute value of the feature quantity is greater for the fields created from different frames, and the correlation between the fields is smaller. Thus, if the difference absolute values between a field and the repeated field of the field are plotted in a distribution chart as shown in FIG. 10, then the plotted difference absolute values are distributed in a range equal to or smaller than a threshold value a.
As for the fields other than repeated fields, comparing a field with the field two fields before is to compare fields derived from different frames. Thus, since the feature quantity is larger and the correlation is smaller for such the fields, the difference absolute values between fields from different frames are distributed in a range less than or equal to a threshold value b or in a range greater than or equal to a threshold value c.
In the 2:3 pulldown sequence detection, the field which includes the feature quantity distributed in the range that is less than or equal to the threshold value a is determined as the repeated field. Further, the field which includes the feature quantity distributed in the range that is less than or equal to the threshold value b and greater than or equal to the threshold value c is determined as a field other than the repeated field, thus detecting the regularity of occurrence of the repeated field once in the five field cycle.
In conventional technologies that are mentioned earlier, the frame difference or the field difference (the feature quantity) is calculated to calculate the movement of the image and a calculation result is compared to the fixed threshold values to detect the repeated field. However, in an image that includes less amount of movement or an image that includes only local movement (for example, a blinking movement), the difference absolute value of the feature quantity (the field difference) decreases and the correlation between the fields increases even for a field other than the repeated field. Due to this, a field other than the repeated field is also determined as the repeated field, thus resulting in a detection error or a detection failure. Moreover, the regularity of occurrence of the repeated field once in the five field cycle cannot be detected accurately and the 2:3 pulldown sequence detection cannot be carried out precisely.